Wide bandgap semiconductor devices are devices constructed from wide bandgap materials, e.g. generally having an energy gap greater than 2.5 eV, such as Silicon Carbide (SiC), Zinc Sulfide, and Gallium, Aluminum, and Indium Nitride related compounds. Wide bandgap semiconductor materials typically include properties that are desirable for constructing power devices, including, among other things, a wide bandgap, a high thermal conductivity, high breakdown field strength, and a high electron saturation velocity. One example of such a power device is a bipolar junction transistor (BJT). BJT's are well-known and frequently used semiconductor devices that are generally defined by two back-to-back p-n junctions formed in a semiconductor material in close proximity. In operation, current enters a region of the semiconductor material adjacent one of the p-n junctions called the emitter. Current exits the device from a region of the material adjacent the other p-n junction called the collector. The collector and emitter have the same conductivity type and include a thin layer of semiconductor material having the opposite conductivity positioned between them, referred to as the base.
One of the requirements for an operable and useful device is an appropriate semiconductor material from which it can be formed. The most commonly used material is silicon (Si), with recent attention being paid to materials such as Gallium Arsenide (GaAs) and Indium Phosphide (InP). While the potential of SiC is recognized, appropriate techniques for producing devices is lacking, because the requirements of specific devices are often difficult to achieve using SiC. For instance, performance optimization in a device, such as a BJT for an RF power amplifier, requires minimizing base resistance, maximizing power densities, and minimizing parasitics. To accomplish these characteristics, the geometry and spacing of the thin base layer and the thicker emitter layer, as well their respective contacts, must be carefully controlled. Furthermore, such devices require careful control of the conductivity and thickness of the layers to achieve desired oscillation frequencies and power gains.
As will be appreciated by those skilled in the art, such careful control of layer thickness requires precise etching. This is especially true in constructing BJT devices from SiC, as the thicker emitter layer must be accurately etched away without removing the thin base layer, which directly influences the device performance. Unfortunately, however, conventional SiC etching methods do not always result in uniform etch times, even where the layer thickness is known, thereby making it difficult to accurately control etching during device construction.